Plasma-mediated cutting of biological tissue with sub-microsecond pulses of high voltage is described in the patent of Palanker (U.S. Pat. No. 6,135,998), herein incorporated by reference in its entirety. Dissection of tissue based on explosive vaporization by short (under a few microseconds) pulses of high voltage is described in the patent of Lewis et al. (U.S. Pat. No. 6,352,535). In these applications, an inlaid cylindrical electrode (i.e., a wire embedded into a thick insulator and exposed at its end) is applied to ionize, evaporate and fragment tissue in proximity of the electrode using dielectric breakdown or vaporization of water induced by a high electric field. An inlaid cylindrical electrode cannot penetrate into tissue and thus can only produce shallow cuts on its surface. Due to the pulsed regime of application, this device produces a series of perforations in tissue, which often do not merge into a continuous cut. In addition, cavitation bubbles accompanying each pulse create substantial collateral damage in tissue during their growth and collapse phases. For example, see “Effect of the Probe Geometry on Dynamics of Cavitation,” D. Palanker, A. Vankov, and J. Miller, Laser-Tissue Interactions XIII, vol. 4617 SPIE (2002). The size of such a damage zone typically far exceeds the size of the electrode and the corresponding zone of initial energy deposition. Reduction in pulse energy helps to reduce the mechanical damage, but may also lead to decreased cutting depth.
A second mechanism of electrosurgical ablation is formation of plasma following vaporization in the proximity of the probe by either a continuous radio frequency waveform or with sub-millisecond long bursts of pulses. For example, see U.S. Pat. No. 6,780,178, herein incorporated by reference in its entirety. This mechanism is universally applicable to soft and hard biological tissue ranging from membranes and retina to skin and cartilage. In such regimes, wire electrodes are typically used, although the use of a device that could provide a uniform electric field along its length would be preferable.
Without considering end effects, the electric field in a conductive medium at a distance r from a cylindrical electrode with potential U and radius r0 much smaller than its length L is:E=U/(r ln(rO/L))  [1]assuming that the return electrode is much larger and positioned at infinity. The threshold electric field required for dielectric breakdown in water is on the order of 105-106 V/cm (Jones, H. M. & Kunhardt, E. E. Development of Pulsed Dielectric Breakdown In Liquids. Journal of Physics D-Applied Physics 28, 178-188 (1995); Jones, H. M. & Kunhardt, E. E. Pulsed Dielectric Breakdown of Pressurized Water and Salt Solutions. Journal of Applied Physics 77, 795-805 (1995)). Such a threshold electric field Eth can be achieved with electric pulses of several kV on a wire electrode with a diameter of several tens of micrometers. The threshold voltage required for ionization of a surface layer of water is:Uth=EthrO ln(L/rO)  [2]
The corresponding threshold energy is:Fth=2πEth2rO2L ln(L/rO)  [3]Evaporation of water in the proximity of an electrode begins when the temperature is elevated above 100° C. The threshold voltage required for vaporization of a surface layer is:Uth=(cρΔT/(τγ))1/2rO ln(L/rO)  [4]where τ is a pulse duration, γ is the electrical conductivity of the liquid, ρ is the liquid density, c is the liquid heat capacity, and ΔT is the temperature change. The corresponding threshold energy is:Fth=2πcρΔTrO2L ln(L/rO)  [5]
Lower threshold voltage and energy, as well as better localization of energy deposition can be achieved by decreasing the radius of electrode r0, as follows from equations 1-5. However, this approach is limited by the mechanical strength of the thin wire and its visibility. In addition, the problem of non-uniform distribution of electric field along the electrode, and particularly, enhancement at the apex remains.
This enhancement is illustrated in FIG. 1A, which shows the electric field surrounding a wire electrode. The field is stronger at the apex (i.e., at distance=0) and is weaker in its cylindrical portion. Thus, ionization and vaporization on such an electrode will always begin and be dominant at locations of enhanced field strength, leading to uneven cutting and excessive damage in front of these singular points, as shown in FIG. 2.
One geometry that provides uniform enhancement of an electric field is a ring electrode shown in FIG. 3. Its field is uniform except for the points of deviation from perfectly round shape, such as where the ring electrode contacts with a holder. Fortunately, these regions of deviation can be kept away from tissue during surgery. The threshold voltage on such an electrode is set by the wire radius (e.g., see equations 2 and 4) and thus may be limited by the mechanical strength of the wire. For example, a thin wire is very weak and flexible and is thus inapplicable to manipulation of tissue. In addition, wires thinner than 25 microns are barely seen under a conventional surgical microscope, and this makes their use even more difficult. An additional problem with the application of thin wires is that erosion of thin wires greatly limits their lifetime.
Erosion of the blade may occur because of electrochemical or thermal reactions on the electrode, and may be a problem for existing electrosurgical electrodes, including electrosurgical blades. Such blades typically have flat sides and an exposed active edge. During electrosurgical cutting with plasma, the plasma formed along the exposed electrode surface may result in localized high temperatures that may differentially etch the electrode and the adjacent insulation. The result is to change the geometry of the cutting electrode, which may be particularly undesirable, and may affect the ability to cut with the electrode, as well as the energy required to drive the electrode.
For example, FIGS. 10A to 10C illustrate differential erosion of electrodes. FIG. 10A shows an initial cross-section through a cutting electrode. The conductive metal region of the electrode 1001 is surrounded by insulation 1003, except at the exposed tip 1005. When this electrode is to be used with electrosurgical (plasma) cutting, appropriate electrical stimulation may be applied to the electrode so that plasma is formed at the tip 1007. If the vaporization or melting temperature of the insulation 1003 is less than the temperature reached by the plasma (e.g., approximately 800° C.), the insulation may be removed from the electrode, as shown in FIG. 10B. In FIG. 10B, the insulator has retreated from the cutting region of the electrode during activation of the plasma (e.g., plasma-mediated electrosurgery), exposing the conductive metal 1001, which may lead to an increase in electric current flowing from the electrode into the conductive medium or tissue. This results in higher power dissipation which may lead to increase generation of heat in the tissue volume, formation of excess gas (bubbles), an increase in the zone of electroporation damage, and unstable generation of plasma during surgery. This problem may be a result of insulation materials having a low-melting and/or vaporization point (e.g., volatile insulators such as plastics). Such materials may vaporize or erode faster than the metal electrode during a plasma mediated electrosurgical application, as shown. However, a parallel problem may result when the insulation layer is too resistant to erosion, as shown in FIG. 10C. In this example, the insulation does not erode as rapidly as the metal electrode erodes. As the conductive metal erodes from the insulation, a gap is formed between the metal, resulting in a gap between the conductive metal and the surrounding material. At some point this gap may prevent cutting of tissue by the electrode, either because of physical separation from the tissue, or because the voltage will be insufficient for vaporization and ionization, terminating the electric discharge.
Below we describe probe geometry and pulse waveform structures that provide solutions to these and other problems.